In a beam scanner a small spot of light is swept rapidly across the target. After reflection from the target a photo electric converter such as a photo diode detects the reflected light and converts it to electronic signals representing features of the target. To successfully resolve features of the target, it is necessary that the spot size be about the size of, or smaller than, the smallest features of the target
A bar code reader is an important commercial application for beam scanners and is referred to herein as a typical example of a specific application for the present invention.
In typical barcode scanners, optical components such as converging lenses and stop apertures are placed in front of a light source such as a laser diode causing the beam to converge to a small spot about several inches away from the source. The densest bar codes, those with the narrowest bars and spaces, are most resolvable where the beam spot has the smallest cross sectional dimensions. This narrow region is sometimes referred to as the beam waist.
The range of distance over which the most dense codes may be resolved with a fixed focus lens is quite short. For example dense bar code targets with 5 mil, (0.005 inch), wide bars and spaces may only be resolvable over a range of 1 or 2 inches when the waist is located at about 6 inches from the laser source. Such readers are perceived to have a sensitive "sweet spot" when attempting to read dense codes.
Beyond the beam waist where the beam diverges it is only possible to resolve wider bars and spaces. It is very desirable however for portable scanners to have a longer depth of operating field for the dense 5 and 7 mil bar codes.
In the U.S. patent application entitled "ELECTRO-OPTICAL SCANNING SYSTEM WITH GYRATING SCAN HEAD", Ser. No. 07/776,663 of which the present invention is a continuation-in-part, several methods were described for extending the depth of field of a beam scanning bar code reader. One of these methods employs the use of a moving lens system which may be electrically focused and another employs a non-imaging cone shaped optical element to provide a beam which is narrow over a certain distance then diverges rapidly beyond that distance.
U.S. Pat. No. 4,816,660 describes the use of a conventional aperture stop to increase depth of field of a laser bar code reader. The aperture stop has draw backs in that it wastes a substantial portion of the laser beam power to gain increased depth of field and requires precise mounting of numerous parts including a separate blocking wall, aligned with and spaced from a separate lens, thus forming a bulky system.
For fixed focused systems, when the beam spot is not at its smallest size at a particular distance down range, a dense bar code target becomes difficult or impossible to resolve.
U.S. Pat. No. 5,438,187 describes lenses with compound surfaces, i.e., different curvatures on a single lens surface to simultaneously focus light at different points down range. This approach is difficult to implement because the production of such lenses is complicated even if they are molded from plastic. The focusing of such lenses is also complex and leads to tedious compromises. Signal processing is also impaired due to decreased signal to noise ratios associated with simultaneously apportioning light to different regions. (Light not in focus for one region may add to noise especially between foci.)